1. Field of the Invention
The present invention relates to transmitting apparatus with an interface for network-to-network communications, and a frame transfer method utilizing such transmitting apparatus, and more particularly to transmitting apparatus and frame transfer method capable of simplifying learning functions without degrading the frame transfer performance.
2. Description of the Related Art
A layer 2 switch (hereinafter, “L2 switch”) has a function of learning path information and independently deciding a transfer destination of a Media Access Control (MAC) frame. More specifically, the L2 switch learns in advance the relationship between the source MAC address of the MAC frame and the position of the receiving port, so as to transmit, if the destination MAC address in the MAC frame proves to be a learned one upon receipt of the frame, the MAC frame only to the designated port based on the learned information.
The function of learning the path thus eliminates the need to reproduce the MAC frame for all the ports, thereby preventing generation of useless traffic. Such path learning is also performed in an Asynchronous Transfer Mode (ATM) network or the like where a plurality of networks is connected, as disclosed, for example, in Japanese Patent Application Laid-Open No. H8-18571.
The recent progress in communications speed of networks requires quicker operations in deciding the paths from transmitting apparatus such as the L2 switch, and hence the learned information of the paths is often stored in a Content Addressable Memory (CAM) which allows quick memory retrieval.
However, employing the CAM results in a higher cost of the transmitting apparatus because the CAM is an expensive memory. Especially in chassis-type transmitting apparatus that includes a plurality of communications units, many CAMs have to be employed because the path information has to be stored in each communications unit, and therefore, the cost increases accordingly.